<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style type="text/css">
    body {
      padding: 0;
      margin: 0;
    }
    #box1 {
      width: 400px;
      background-color: aqua;
      display: flex;
    }
    #child1 {
      width: 40%;
      background-color: blueviolet;
    }
    #child2 {
      width: 60%;
      background-color: antiquewhite;
    }
  </style>
</head>
<body>
  <div id="box1">
    <div id="child1">ss</div>
    <div id="child2">mm</div>
  </div>
</body>
</html>
